With compact structure, PCB winding transformer is now widely used in switching mode power supplies, wherein it requires high power density and/or low profile form factor. Basically, a PCB winding transformer consists of at least a primary winding and a secondary winding, and there is parasitic capacitance between them, which impact EMI performance seriously. The present disclosure is devised to control the parasitic capacitance mentioned above, so as to improve the EMI issue.